Mechanical dynamometers are often used for static and dynamic measurements of mechanical forces. The forces to be measured may cause changes in the electrical charge, voltage, current or impedance in one or more measuring elements.
When measuring mechanical vibrations or acceleration it is known to use seismic accelerometers employing piezoelectric material for generating the electrical charges. For such accelerometers it is known to have a seismic mass arranged on the piezoelectric element or material which again is arranged on a base. Thus, when the accelerometer is subjected to acceleration, inertial forces introduce strains in the ceramic element which produces electrical outputs by virtue of the piezoelectric effect.
When vibrations having a frequency which is substantially lower than the natural resonant frequency of the total accelerometer system are acting upon the base, the seismic mass is forced to follow the vibrations thereby acting on the piezoelectric element with a force which is proportional to the seismic mass and the acceleration. Thereby, the inertial force acting on the piezoelectric element generates electrical charges on the element which charges are proportional to the acceleration.
When the piezoelectric element is subjected to compression forces during vibration, the accelerometer is of the compression type, and when the piezoelectric element is subjected to shear forces during vibration, the accelerometer is of the shear type. The compression type accelerometer is the most simple in its construction, but it is rather sensitive to temperature transients since the ceramic piezoelectric material is pyroelectric in the axis of polarization and the signals is taken from electrodes perpendicular to this axis. In contrast to this, the shear type accelerometer has a low sensitivity to temperature transients as the signal here is taken from electrodes parallel to the axis of polarisation.
It is known that a higher sensitivity can be obtained by an accelerometer of the "bender" type. In such an accelerometer, the force from the seismic mass acts to bend a so-called "bender element", which has a layer of an electric conductive material sandwiched between two layers of piezoelectric material being polarized in their direction of thickness. Thus, when the element is bent, then, in a plane perpendicular to the longitudinal axis of the element, stresses of compression are generated in one of the two layers and stresses of tension are generated in the remaining layer. When the length of the bender element is considerable larger than the thickness of the element, the electrical charges generated on each of the two layers will be larger than the charges obtained if the same seismic mass is operating directly for the purpose of compression or shear of the piezoelectric material.
However, a disadvantage of the bender element is that it is pyroelectric, since the electrodes are arranged on surfaces which are perpendicular to the axis of polarization.
Another disadvantage of the bender element is that the piezoelectric material constitutes a major part of the mechanical construction which causes some problems when trying to optimize this construction.